COMPLEX STRUCTURE OF OLD SHIELDS 73 



rocks. When these rocks of the basement are again uphfted in a later 

 period and the covering of younger sediments is eroded away, the 

 basement rocks will not show too high a metamorphism, not too 

 strong a recr\stallization of their constituent minerals. So it is only 

 in the anciently stabilized parts of the crust that we may hope to 

 find undisturbed fossil remains of earlv life. 



An example of this difference in metamorphism between rocks 

 of similar age according to whether they have been laid down in a 

 geosynclinal basin of an orogenetic cycle or on a stable platform, 

 can be found in the European Cambrian. All around the old shield 

 of Fennoscandia, the core of the European continent, sediments were 

 laid down from the Cambrian to the Silurian. In Noru^ay and 

 Scotland a geosynclinal basin formed, which in turn developed into 

 the Caledonian orogeny, with very thick series of sediments. Farther 

 east much thinner series were deposited on the stable pre-Cambrian 

 basement. The rocks in the Caledonian foldbelt are all more or less 

 strongly metamorphosed, whilst those on the stable platform were 

 affected so little that around Moscow bricks are made of soft clays 

 of Cambrian age. 



COMPLEX STRUCTURE OF OLD SHIELDS 



In contrast to the impression evoked by the uniform stippling by 

 which the old shields are indicated in many geology textbooks, they 

 are built similarly to the younger regions of the crust. In the shields 

 too, a succession of older and younger orogenetic belts can be found, 

 whilst part of the areas of preceding orogenetic cycles have no longer 

 been mobilized by subsequent orogenies. The old shields consequently 

 are not a simple and uniform area forming the core of later struc- 

 tures. They themselves consist of older and still older cores, sur- 

 rounded, and sometimes also cut up, by newer orogenetic belts. This 

 is indicated in Fig. 15 where a number of absolute dates is given for 

 various points of these shields. It is seen at a glance that the ages of 

 various parts of the shields, often in quite close proximity, can show- 

 very great variations. A certain number of the major pre-Cambrian 

 orogenies is indicated in Table III. 



Of course, the further back we go into the history of the earth, 

 the smaller are these oldest cores, which were already stabilized bv 



